2. Introduction
Zero electrical resistance
◦ Superconductors carry current without energy loss
Perfect diamagnetism
◦ Superconductors float (levitate) above magnetic fields
3. History of superconductors
1911: Onnes finds that at 4.2K the
resistance of mercury suddenly
drops to zero. He called this effect
superconductivity and the
temperature at which this occurs,
critical temperature Tc
1933: Walter Meissner and Robert
Ochsenfeld discover that a
superconducting material repels a
magnetic field (Meissner effect)
4. History contd.
1957: First widely-accepted theory by John Bardeen, Leon
Cooper, and John Schrieffer (BCS theory)
1962: Brian D. Josephson predicts that electrical current
would flow between two superconducting materials - even
when they are separated by a non-superconductor or
insulator. “Josephson effect”.
1986: Alex Müller and Georg Bednorz created the first
superconducting cuprate: La2-xBaxCuO4 (Tc =30 K). Got
Nobel in 1987. “High Tc superconductivity”
1987: Discovery of YBa2Cu3O6+ (YBCO) a material that
superconducts at temperatures above the temperature of
liquid nitrogen - a commonly available coolant
The current world record Tc of 138 K is held by
Hg0.8Tl0.2Ba2Ca2Cu3O8.33
5. Perfect diamagnet & superconductor
Perfect diamagnet Superconductor
If a conductor already had a Remarkably, the magnetic
steady magnetic field through it behavior of a superconductor is
and was then cooled through the distinct from perfect
transition to a zero resistance diamagnetism. It will actively
state, becoming a perfect exclude any magnetic field
diamagnet, the magnetic field present when it makes the phase
would be expected to stay the change to the superconducting
same. state.
Two mutually independent properties defining SC are = 0 and B =
6. Effect of magnetization
Superconductivity can be destroyed also by an
external magnetic field Hc which is also called the
critical one
Phase
diagra
m
2
T
HC H C (T 0) 1 2
TC
7. Types
There are two types of superconductors, Type I and Type
II, according to their behaviour in a magnetic field
Type I
superconducting state
normal state
This transition is
abrupt
Type I superconductors are pure metals and alloys
8.
9. Type II
superconducting normal state is gradual
10. Types I & II comparison
The Type II superconductors
have much higher critical
magnetic fields than Type I, but
for most of that field range they
are mixtures of normal and
superconducting.
12. BCS Theory
BCS Theory (1957) deals with the behaviour of electrons in
superconducting materials at very low temperatures
Low temperatures minimize the vibrational energy of individual
atoms in the crystal lattice
An electron moving freely through
the material encounters less
impedance due to vibrational
distortions of the lattice at low
temperatures
The Coulomb attraction between
the passing electron and the
positive ion distorts the crystal
structure
13. The region of increased positive charge density propagates
through the crystal as a quantized sound wave called a phonon
2 1
- -
The passing electron has emitted a phonon
A second electron experiences a Coulomb attraction from the
increased region of positive charge density created by the first
electron
14. BCS Theory contd.
Electrons are said to pair into Cooper pairs through interaction with
the crystal lattice (indicated by isotope effect where TC is different for
different isotopes)
Cooper pairs are formed by two electrons, which overcome their
Coulomb repulsion and experience an attraction through phonon
exchanges
Cooper Electron Pairs act like single particles (BOSONS)
The electrons in a Cooper Pair possess antiparallel spin, resulting in
a total spin of zero for the pair
Since the Cooper Pair has zero spin, the pair is not required to obey
the Pauli exclusion principle
Bosons are particles which have integer spin and their energy
distribution is described by Bose-Einstein statistics
15. Condensation: At low temperatures, bosons collect into the same
energy state
Cooper pairs condense into a highly ordered ground state
The pairs retain this ordered structure while moving through the
crystal lattice
Each pair becomes locked into its position with others pairs, and as
a result no random scattering of electron pairs may occur
Zero resistivity may be defined as the absence of electron
scattering; hence, the superconductor now demonstrates zero
resistivity
The binding energy of a Cooper pair at absolute zero is about 3KTC
As the temp rises the binding energy is reduced and goes to zero
when T=Tc. Above T=Tc a Cooper pair is not bound.
16. Findings of BCS theory
The binding energy of Cooper pair gives arise to
energy gap of the order of 10-3 eV
Eg(T = 0) = 3.53 kTC
17. Applications
• Diamagnetism
The wide applicability of
• Zero resistance
superconductors is due to
• Higher current
Medical Industry
MRI Exploits the high
magnetic fields expelled
by superconducting wires
for medical applications
Since the superconducting coils are capable of producing very stable,
large magnetic field strengths, they generate high quality images.
18. Transportation Industry
Superconductor coils create strong magnetic fields that produce
the effect of levitation by repulsion
Maglev trains hover above a magnetic field without any
contact with the tracks
As a result, high speeds of up to 500 miles per hour are
possible with only a small consumption of energy
19. Electric power industry
High temperature superconductors (HTS) can be used in the
production of more cost effective motors and generators
HTS power cables can carry Superconducting cyclotron
two to ten times more power (MSU)
in equally or smaller sized
cables
20. References
A. Beiser – “Concepts of Modern Physics”, 6 Ed., Tata
McGraw-Hill (New Delhi, 2003)
Charles Kittel – “Introduction to Solid State Physics”, 7
Ed., John Wiley and Sons (New York, 1996)
www.wikipedia.org
http://hyperphysics.phy-
astr.gsu.edu/hbase/hframe.html
